Calculated Effect of Alloy Additions on the Saturation Magnetization of Fe 0.80 B 0.20
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Calculated Effect of Alloy Additions on the Saturation Magnetization of Fe0.80B0.20 D. M. C. Nicholson1, Yang Wang2, and Mike Widom3, 1 Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 2 Pittsburgh Supercomputing Center, Pittsburgh, PA 15213 3 Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 ABSTRACT The great number of different local environments in amorphous alloys leads to the evolution of complicated non collinear magnetic structures. Alloy additions can affect the magnetic structure in surprising ways. For example, replacement of a small amount of Fe with Co increases the saturation magnetization even though Co has a much smaller moment than Fe. The calculated behavior of the magnetic structure of (Fe(1-x)Mx) 0.8B0.2 with M=Co, Cr, Zr, and Mn2Zr are presented.
INTRODUCTION For many years rapidly quenched iron based metallic glasses have been of technological importance because of their high permeability. The discovery in the past decade of alloy compositions that can form bulk metallic glasses by slow cooling from the melt has generated renewed interest in magnetic glasses. In this paper we describe our efforts to construct realistic models of the magnetic structure based on first principles calculations. For amorphous materials the importance of such models is elevated because of the difficulty of experimental structure determination. Calculations of the magnetic structure based on these structural models give values of the saturation magnetization that can be compared to measurements. These comparisons serve to corroborate the validity of the structural models. TECHNIQUE Our investigation begins with models of the well studied the near-eutectic glass Fe0.80.B0.20 to which we add Co, Cr, Zr and Mn2Zr . We employ small structural models consisting of 100 atoms in a cubic cell with periodic boundary conditions. The structures are generated using the Vienna ab-initio simulation program (VASP) [1], a pseudo potential based molecular dynamics code, to obtain the liquid state. Rather than ignore local magnetic effects in the simulation of the liquid we treat the liquid as a collinear magnetic system. This treatment permits ferromagnetic and anti-ferromagnetic alignment of spin moments. This approximate inclusion of magnetic effects is found to improve the agreement between, for example, the measured and calculated pair distribution functions of liquid iron.
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VASP molecular dynamics was performed for Fe0.80.B0.20 to establish equilibrium at 1400K. The system was instantaneously quenched to the local metastable equilibrium configuration. Calculations were performed at 'medium' precision in the terminology of the VASP software; one “K-point” in the Brillouin zone was used. These choices are typical of MD for this number of atoms and give a reasonable balance between accuracy and computation time. Forces were calculated with more “K-points” for a few configurations to confirm that “K-point” convergence would not compromise the val
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